In this investigation, the possible role of the electrogenic sodium pump in slow synaptic inhibition was analyzed. The experiments were carried out on the ninth or tenth paravertebral sympathetic ganglia of the bullfrog using the sucrose gap technique. We found that although electrogenic sodium pumping was inhibited by ouabain or potassium-free Ringer, the slow IPSP was not blocked by ouabain and was enhanced in potassium-free Ringer. These data indicate that the slow IPSP is not generated by the synaptic activation of the electrogenic sodium pump.